few more details can help. eg-- is it the strength in knees? or the technique of actually bending and then uncoiling?

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What he asked ^^ and...try to stand on your toes *front half of your feet* rather than flat footed. Try standing upright without bending anything yet. Then stand on your toes as if you are trying to look taller. Then keep everything the same except this time bend your knees too. This should be the "coiling" motion before impact with the ball. Then you just naturally straighten your ankles and knees out right before the moment of ball impact.

Serving with the method above is considered "perfect" when you hit the ball right as you reach the apex of your jump (your maximum height). This delivers the most power with the least effort from your arm.

i dont get it... just bend your knees. unless you are doing one of those bending the knees to toss, then trying to bend the knees again before you hit the ball type deals. which is really weird and unnatural to me and also confusing as to why some ppl do/try it.